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DOVER – The lead public defender for accused murderer Seth Mazzaglia sought to convince the jury in his closing argument Wednesday morning that Mazzaglia’s ex-girlfriend, Kathryn “Kat” McDonough, is a lying, scheming manipulator who has had every motivation to cover up her alleged role in the October 2012 death of UNH student Elizabeth “Lizzi” Marriott.

“What we have is Kat McDonough being the manipulator of facts, and lying to Seth and to you,” defense attorney Joachim Barth said in Strafford County Superior Court, where every row in the courtroom was filled at the conclusion of the highly emotional trial that saw 19 days of testimony after opening statements May 28.

Barth took more than the allocated two hours to make his final case. He spent nearly all of that time focusing on McDonough, 20, whom he accused of crafting an elaborate story about Marriott’s death and events in preceding months “to distance herself from the truth, and (from) the truth that may endanger her severely.”

Barth attacked McDonough’s credibility and reliability from numerous angles, mimicking her tentative, quavering voice on the stand at least twice. He reminded the jury about Facebook messages in which she gleefully gloated about leaving bite marks on Mazzaglia’s neck and bruises on his chest during sex – despite testifying in court that Mazzaglia dominated her in an abusive, controlling relationship – and cited McDonough’s “cold, calculated lies” to police, whom she initially told that Marriott had never arrived, on the night of her death, in the one-room Dover apartment McDonough and Mazzaglia shared.

But before all of that, Barth told the jury that he was really going to speak, in sum, about “the only issue in front of you.”

“I want to talk to you about doubts,” he said.

Mazzaglia, 31, faces first- and second-degree murder charges in the Oct. 9, 2012, death of Marriott, a 19-year-old from Westborough, Mass.

Prosecutors allege Mazzaglia strangled Marriott to death with a white cotton rope after she rebuffed his sexual advances after a game of strip poker in the Dover apartment.

Defense attorneys have accused McDonough of killing Marriott that night during rough sex that led to suffocation and a seizure.

Barth alleged that as she lied to a grand jury in February 2013 and in Mazzaglia’s trial this month, McDonough created a story that was based on “four pillars:” her claim that she was submissive to Mazzaglia in an abusive relationship; that he entirely dominated their sexual relationship; that he instigated and led the couple’s pursuit of other women to join them in sex acts involving bondage; and that Mazzaglia has other personas that are dark enough to spur homicidal rages, such as the one that McDonough said took over that night.

He cited text and Facebook messages in which McDonough wrote “WIN!!!” and the sinister-sounding “mwahaha” when chatting with Mazzaglia about their own sex and possibilities for bringing in other women.

“Did she appear to you to be someone who is submissive and weak?” Barth asked.

He also cited last week’s testimony by state police Sgt. Sara Hennessey, who said she interviewed McDonough in a Newington police station on Oct. 12, 2012, after meeting her and Mazzaglia outside the Best Buy in Newington where Mazzaglia worked.

Hennessey said McDonough told her and another officer that she hadn’t seen Marriott on Oct. 9, 2012, despite their plans to shoot photos in a Dover cemetery. Hennessey said McDonough told them she instead went to the cemetery alone. McDonough initially described a cool night on which “the moon was peeking through every once in a while” and she had to change her camera settings several times.

He noted that the officers told McDonough “that Seth was talking,” and reminded the jury that McDonough had testified about how she and Mazzaglia had crafted the “sex gone wrong” story as a “Plan B.”

"Shouldn’t that have been a really clear green light to invoke Plan B?” Barth said, telling jurors that if Sgt. Hennessey and another officer had told McDonough that Mazzaglia was talking, that would have seemed like an ideal time to invoke the story McDonough said was their planned lie.

"Do you see the importance of the fact that she doesn’t adopt Plan B?” Barth pressed. “She should have, right there – if there really was a Plan B.”

Barth noted that according to Hennessey, McDonough’s response when they told her Mazzaglia was talking was, instead, an involuntary reaction.

“They said blood drained from her face,” Barth said. “It was Kat McDonough who was worried that Seth Mazzaglia was going to talk – why?”

McDonough has testified that after Mazzaglia strangled Marriott, she helped Mazzaglia pack Marriott’s body into a suitcase and drive Marriott’s car to Peirce Island in Portsmouth, where McDonough said they pushed Marriott’s body into the water. Marriott’s body has never been found.

McDonough is serving a 1½- to three-year prison term after pleading guilty last July to charges that included witness tampering and hindering the investigation, as part of a plea agreement. She has testified that she helped cover up the murder and rape of Marriott.

Barth asked the jury Wednesday to be careful about how they perceived McDonough’s immunity, which was granted through the plea deal and which prosecutors have said frees her to tell the truth about Marriott’s death.

Barth said McDonough still has motivation to lie and cover up her alleged role, which he said still could create consequences for her.

“They can’t use what she said to the grand jury, but they can use any other evidence,” he said of law enforcement. “They can’t use her words, but it ain’t a ‘get of jail free’ card.”

He said McDonough could face years in prison if her alleged role in Marriott’s death, or her alleged lies in sworn statements, came to light.

“She has years of motivation. She has all the motivation of the beautiful things that life has to offer,” Barth said. “She has fear as a motivation. She has ‘it’s all about Kat’ as a motivation.

“She needs this more than anyone,” Barth said earlier in his argument. “You don’t think she’s willing to lie?”

The trial continues Wednesday afternoon, with the prosecution’s closing argument.

In his final statements to the jury, Barth offered a preview of what could come.

“The state will hold up a photograph of Liz Marriott – it will remind you of the magnitude of this case,” he said. “That moment will provoke passions.”

He asked the jury to remember, in that moment, principles of justice.

“Justice in this case certainly requires you to examine the source of evidence,” he said, pointing to the witness stand, where McDonough gave nearly 10 full days of testimony earlier this month.

“Recognize your doubts about this claim, (and) recognize your doubts about the source of this claim,” Barth said.